Elevated plasma triglycerides (TG) increase the risk of cardiovascular disease, a leading cause of morbidity and mortality in the United States. As the primary determinant of plasma TG levels, lipoprotein lipase (LPL), anchored by the endothelial cell protein GPIHBP1 to the capillary luminal surface, hydrolyzes circulating TG into free fatty acids that are taken up by peripheral tissues. LPL activity is tightly regulated to enable circulating TG be routed into cardiac and skeletal muscles to generate energy during fasting and into white adipose tissue (WAT) for storage in the fed state. However, the molecular mechanisms for regulating tissue-specific LPL activity during the fed-fast cycle are largely unknown. ANGPTL8 (lipasin) is a recently described TG metabolism regulator, which is specifically expressed in liver and adipose tissues. In the liver, which secretes ANGPTL8 into the circulation, ANGPTL8 expression is reduced by fasting and dramatically increased by feeding. In mice, ANGPTL8 deficiency causes hypotriglyceridemia, whereas its hepatic overexpression causes hypertriglyceridemia. Despite having great therapeutic potential in treating dyslipidemia, ANGPTL8?s functions and mechanism of action remain elusive. Based on our solid preliminary data, we propose to study the functional roles of ANGPTL8 and the mechanism by which it regulates TG metabolism. In Aim 1, we will determine functional roles of ANGPTL8 in regulating TG trafficking, by performing fluxomics analysis with isotope-labeled TG in ANGPTL8 KO mice, wild-type mice with injection of an ANGPTL8 monoclonal TG- lowering antibody, as well as liver and adipose-specific KO mice. In Aim 2, we will elucidate mechanisms of ANGPTL8-mediated inhibition of GPIHBP1-bound LPL. By using a novel cell-culture system to examine LPL complexed with GPIHBP1 on the endothelial cell surface, we will examine the interactions among ANGPTL8, ANGPTL3, and LPL-GPIHBP1 complexes. Upon completion, we will have revealed the molecular mechanism by which ANGPTL8 regulates LPL to direct TG trafficking to specific tissues during the fed-fast cycle, a physiological process of fundamental importance.